Apparatus and method of processing and drying lumber and similar material



H. w. COWAN 1,887,581

APPARATUS AND METHOD OF PROCESSING AND DRYING LUMBER AND SIMILAR MATERIAL Nov. 15, 1932.

Filed Deb. 51. 1930 ljhvemror I Cowah ATTK Patented Nov. 15, i932 HENRY WARRINGTON COWAN, F TORONTO, ONTARIO, CANADA.

APPARATUS AND METHOD or PROCESSING AND DRYING MATERIAL LUMBER. AND smmaa Application filed December 31', 1930. Serial No. 505,844..

In the practice of drying lumber or other material requiring similar treatment, it is usual to enclose the material within a housing or kiln where it is subjected to heat to turn the moisture within the material into vapor. In drying material such as lumber extreme care must be taken to ensure that the condition known in the trade as case hardening does not occur. This case hardening,

which is simply a sealing of the pores in the outer part of the materlal, prevents the es cape of moisture from the interior of the material, and is due to heating the material too rapidly, causing a drying of the outer.

part of the material before the moisture has a chance to escape from its interior. To overcome this difiiculty, it has been usual to inject sprays of steam into the kiln to keep the outer part of the material moist, the steam 2o usually having a high temperature, say, for example 200 degrees. As such spraying is done soon after the lumber is placed in the kiln and the interior of the kiln and the lumber are therefore at a comparatively low temperature, say at not more than 70, it will be apparent that the injection of the high temperature steam into contact with the comparatively cool lumber, subjects the latter to very great strain.

For the removal of vapor from the kiln, the latter is provided with a series of vents or ducts which are manually controlled. 'It is very difiicult, of course, to evenly control these vents, and further, 'as'the heated air tends to circulate vertically, there are marked 1vgaiil'iations in drying at difierent parts of the My object is to devise a method of drying which will avoid all tendency of the material 40 to become case hardened, which will utilize the moisture discharged from the material for this purpose, thus avoiding the injection of steam of high temperature into the kiln, and which will avoid any marked variation of tem erature at different parts of the kiln when t e vapor is discharged from the kiln. A further object is to devise apparatus which will satisfactorily carry out the above process. w

I attain myobject by carrying out my imthe material reaches a predetermined maximum temperature, the air being circulated first at a comparatively low temperature untilthe interior of the kiln at the inlet end attains a given temperature, then at a higher temperature until the kiln adjacent the inlet end attains said higher temperature, and so on in stages until the interior of the kiln attains the desired maximum temperature. Due to the passage of the air over the material and the absorption of moisture therefrom, the interior of the kiln adjacent the outlet end will, during the processing, be-

somewhat lower in temperature than at the inlet end, but in the final stage of processing the circulation of heated air through the kiln will be continued until the interior of the kiln is at substantially the same temperature throughout. Means controlled by the so temperature of the kiln adjacent the air inlet end are provided for automatically controlling the heating means for the air to ensure'that the air is not heated at any time beyond the limit for each stage in the processing.

During this stage by stage processing the moisture passing from the material is retained within the kiln for the purpose of fpreventing undue drying of the outer part 0 the material with its consequent sealing of the pores.

The temperature of the interior of the kiln throughout as well as the material within the kiln having been brought to the desired maximum temperature, processing is thus complete and the drying operating then commen'ces. Drying is effected by discharging moisture laden air from the kiln in comparatively small, amounts -while allowing dry air 1 to replace the discharged moisture laden air. This discharge is effected through a damper controlled outlet. The damper is automatically opened when the temperature at the air exhaust end reaches the same temperature as the air inlet end. The damper control mechanism is such that the damper will close before the temperature of the interior of the kiln has dropped to any material extent, that is to say, from the time the damper opens un: til it closes, the temperature of the interior of the kiln at the outlet end will drop only probably about four or five degrees. The temperature at said outlet is then again raised to the desired temperaturewhen the damper control mechanism again comes into operation, to open the damper and again shut it after a further small-quantity of moisture laden air has been exhausted and dry air taken in to replace it. This series of operations is continued until the material is sufficiently dried.

As a result of this taking in of dry cooler air, the air being no longer saturated with Water vapor, water in the pores of the wood is vaporized and picked up by the air working in from the outside.

The process and apparatus are hereinafter more specifically described and illustrated in the accompanying drawing in which Fig. 1 is a longitudinal section of a more or less diagrammatic nature illustrating apparatus for carrying out my process;

Fig. 2 adiagr'am illustrative of the process employed.

In the drawing like numerals of reference indicate corresponding parts in the different figures.

1 indicates a housing provided with a door 2 at one end for the entrance and exit of the material to be dried. I show the material 3 as piled upon trucks 4. An air conduit 5 is shown leading from one end of the kiln to the other, a fan 6 being shown in said conduit for causing a positive circulation of air. Also located in said conduit is a heating coil 7. A

[valve 8 controls this heating coil, which-valve in turn is adapted to be opened or closed through the instrumentality of an electromagnet 9 operating on a lever 10 connected to the valve operating handle. The circuit of this electromagnet 9 is controlled by a ther-.

mostat 11 or other temperature-responsive instrument located adjacent the air inlet end of the kiln.

The housing 1 is provided with an opening 12 leading to the atmosphere and serving both as an inlet and outlet. This opening 12 is provided with a control damper 13. This damper is actuated by an electromagnet or similar device 14, the circuit of which is controlled by a thermostat 15 or other suitable temperature-responsive device.

The process is carried on as follows. The

' lumber or other material to be dried having been placed in the kiln, the fan 6 is set in operation to cause a circulation of air through the kiln, the air being removed at one end. and returned at the opposite end. The thermostat 15 is set at the maximum temperature to which it is desired to heat the material say 50 F., while the thermostat 9 is set at a low temperature say 7 0 F. The heating coils 7 are normally hot and will continue to be heated until the interior of the kiln adjacent the thermostat 9 reaches F., and this temperature will be maintained under the control of the thermostat until the material is heated throughout to the temperature of the circulating medium. The thermostat 9 is then set for a higher temperature, say 80 degrees, at which'it remains until the interior of the kiln adjacent the thermostat 9 reaches said temperature of ei hty degrees. After the lumber has been su jected to this temperature for some time, the thermostat 9 is reset to a higher temperature, say 90 degrees, and the interior of the kiln then raised to that temperature. The temperature of the interior of the kiln is thus raised step by step until the final temperature of 150 degrees'is reached.

There will, of course, be a lagging behind of the temperature at the exhaust end of the kiln due to the cooling of the air through contact with the material, as indicated in the diagram shown in Fig. 2. The final step of the processing treatment is therefore continued until the temperature of the interior of the kiln throughout reaches 150.

air to escape through the opening 12 and a certain amount of dry air to enter. The temperature within the kiln therefore drops but the thermostat however, is comparatively'quick acting and the damper is closed before the temperature of the kiln is lowered to any appreciable extent. The temperature again is raised to 150 when the damper i again opens, and another quantity of vapor allowed to escape and dry air to enter. This sequence of steps is repeated over and over until the lumber is sufiiciently dry. I

It will be noticed that the lumber is at no time subjected to a temperature much higher than that of the interior of the lumber itself, and is not subjected to any great I strain. Further, it will be noted that on account of the fact that no moisture is ex hausted from the kiln until the lumber has reached the maximum desired temperature, there is always plenty of moisture within the kiln to avoid the necessity of supplying steam to keep the surface portion of the lumber moist to prevent case hardening.

In the case of a remodeled old kiln which it is difiicult to make tightand from which a certain amount of moisture may thus escape, it may be necessasry to supply the kiln with a small amount of steam, but this supply will be very limited and at as low a temperature as possible.

It is difiicult to state at all closely the length of time necessary in the various steps or stages of the process. Much naturaly depends on the type of the heating means, the construction of the kiln, the kind of lumber to be dried and its moisture content. It will be necessary, therefore, as in known processes to inspect the lumber from time to time and make the usual tests to make certain that the lumber is in fit condition before each increase in temperature of the circulating medium.

In the case of a kiln from which there is little or no leakage, it may be necessaryto set the damper thermostat up step by step, particularly in the case of very Wet lumber,

which contains more than enough moisture to saturate the air, so as to evacuate the moisture, as otherwise no drying would be effected.

What I claim as my invention is:

1. A process of drying lumber and similar material which consists in enclosing the material in a kiln forming part of a normally closed circulating system; circulating air through the kiln at a temperature slightly greater than that of the material unti the mass of the material has attained substantially the temperature of the air; raising the temperature of the air in a plurality of stages as the mass of the material reaches substantially the temperature of the preceding stage until a predetermined maximum has been reached and substantially without venting the circulating medium, and continuing the circulation of said medium until there is substantially no tem rature gradient within the kiln between t e ends thereof, then discharging moisture-laden air from said kiln while permitting dry air to enter and thus cool the circulating medium, and reheating the circulating medium to said predetermined temperature and continuing said alternate discharging and heating until the material is sufliciently d 2. A process of drying lumber and similar material which consists in enclosing the material in a kiln forming part of a normally closed circulating system; circulating air through the kiln at a temperature slightly greater than that of the material until the mass of the material has attained substantially the temperature of the air; raising the temperature of the air in a plurality of stages as the mass of the material reaches sub stantially the temperature of the preceding stage until a predetermined maximum has been reached and substantially without venting the circulating medium, and continuing the circulation of said medium until there is substantially no temperature gradient within the kiln between the ends thereof, then discharging moisture-laden air from said kiln while ermitting dry air to enter and thus cool t e circulating medium, and reheating the circulating medium to said predetermined temperature and continuin said alternate dischargin and heating until the material is sufiiciently dry; and regulating the discharge of moisture-laden air and entrance of cool air so that only the outer part of the material is cooled.

Signed at the city of Toronto, Canada, this 16th day of December, 1930.

' HENRY W. COWAN. 

